Model Test-Based Study on Mechanical Mechanism and Design Countermeasures of Capping and Waler Beams During Progressive Collapse of Anchor-Supported Excavations

Local anchor failure can trigger progressive collapse of excavations, during which capping beams and walers, as key load-transferring components, experience significantly increased internal forces. However, the evolution of their mechanical responses remains unclear. In this study, large-scale physical model tests were conducted to systematically investigate the effects of anchor parameters (prestress, failure rate, and installation height), external hazard scenarios (local over-excavation and surface surcharge), and capping beam connection strength on the mechanical responses of capping beams and walers. The results show that applying prestress increases the lateral stiffness of the retaining structure and reduces the bending moment increase in the capping beam. Intermittent instant failure is the most unfavorable condition for the capping beam, inducing larger bending moments than rapid instant failure or slow failure. When anchors are installed at the waler level, the bending moment in the waler is significantly larger than that in the capping beam when anchors are installed at the capping beam level. Local over-excavation subjects the capping beam to larger shear forces at the edges of the over-excavation zone, making it susceptible to shear failure; accordingly, shear strengthening should be implemented at these locations, and strict control over the extent of over-excavation is required. Under surface surcharge, the critical load-bearing component varies with anchor installation height: when anchors are installed at the capping beam level, the retaining piles should be strengthened, whereas when anchors are installed at the waler level, the waler should be strengthened. The wall–anchor support system exhibits superior integrity compared to the pile–anchor system. Capping beam connections effectively disperse failure loads and reduce the increase in axial forces of adjacent anchors. Furthermore, I-steel connections for inter-panel strengthening can further enhance structural stability and increase the number of anchor failures required to trigger progressive collapse. These findings provide a scientific basis for the progressive collapse-resistant design of anchor-supported excavations.